_content/doc/go1.27: note flate compression speed and exact bytes

The improvement to compression speed seems notable enough to be
worth mentioning. (Maybe it can be quantified succinctly enough
for release notes, or otherwise we can point to the issue.)

Even though the exact encoded output is not covered by the Go 1
compatibility promise, it's possible (and given enough scale,
likely) that some Go code or tests to have started to incorrectly
rely on it not changing since it has been a while since the last
change in Go 1.8 (see https://go.dev/doc/go1.8#compressflatepkgcompressflate).
Give a similar heads up, since it's something users updating to
the Go 1.27 toolchain may have to take into account.

For golang/go#75532.
For golang/go#78779.

Change-Id: I5067c42c1f318a722f6325973fa113e9cac06806
Reviewed-on: https://go-review.googlesource.com/c/website/+/791420
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
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